The present invention relates to a drive device comprising a housing, a drive shaft which is supported in the housing and which comprises a shaft axis and a first and second electric drive,                the drive shaft being rotatable about the shaft axis by means of the electric drives and being axially displaceable along the shaft axis,        the two drives being aligned on the shaft axis and each comprising a stator and a rotor,        the stators being fixed to the housing,        the rotors being embodied as hollow shafts and being supported on the housing,        the rotor of the second drive being rotationally locked to the drive shaft by means of a linear bearing device, but being axially displaceable.        
Such a drive device is disclosed, for example, by DE 43 44 335 C2. It is used where a drive shaft is intended to be both rotatable about its shaft axis and displaceable in the direction of its shaft axis. An example of such a use is the movement of a feed screw of an injection molding machine. Examples of other uses are conveyor arms, which serve to convey a workpiece from a first machining station to a second machining station or the like. In the case of these conveyor arms a workpiece is often first raised, then swiveled in an arc and then lowered again. Such movements also occur in the winding of yarns and coiling of wires. Finally such movements are also particularly feasible in drives for lathe spindles.
In the known drive device the second drive is indirectly connected to the drive shaft by way of a threaded spindle, which is in turn rotationally locked to the drive shaft and axially fixed. Arranged on the rotor of the second drive is a drive journal, which engages in a recess in the threaded spindle and is supported in the latter so that it is rotationally fixed but axially displaceable. The rotor of the first drive is rotationally locked to a threaded nut interacting with the threaded spindle and is axially fixed.
The known drive device is complicated to manufacture from a production engineering standpoint, in particular because the recess for the drive journal of the second rotor has to be let into the threaded spindle. Furthermore, the design construction dictates that the two drives be arranged axially in series, giving the drive device a large overall axial construction. Finally the lever arm with which the drive journal of the second drive acts on the drive shaft is relatively small, so that only relatively low torques can be transmitted, resulting in only a relatively low torsional rigidity.